Unit 7, Lesson 7
Building Polygons (Part 2)
Grade 7
Preparation
Lesson Narrative
In this lesson, students experiment with constructing triangles given 2 or 3 side lengths. They start by working with cardboard strips and metal fasteners to discover that there are some combinations of lengths that do not make a triangle. Then students move toward using a ruler and compass, seeing that doing so recreates the function of the cardboard strips and metal fasteners more efficiently. The purpose of this transition is to help students move toward a mental understanding that does not depend on physical objects, helping them work toward the understanding that in a triangle the sum of any two sides must be greater than the other side.
Students use repeated reasoning with specific cases to formulate a general rule about which side lengths are possible for triangles (MP8).
- Explain (in writing) how to use circles to locate the point where the sides of a triangle with known side lengths should meet.
- Use manipulatives to justify when it is not possible to make a triangle with three given side lengths.
- Use manipulatives to show that there is a minimum and maximum length that the third side of a triangle could be, given the other two side lengths.
Let’s build more triangles.
Required Materials
Activity 1
Required Preparation
Each group needs 2 sets of strips and fasteners from an earlier lesson. If needed, prepare additional sets of strips and fasteners, including punching holes in the strips.
For the digital version of the activity, acquire devices that can run the applet.
For the digital version of the activity, acquire devices that can run the applet.